One motivation for dimming a metal halide arc discharge lamp is to provide hot-restart-like performance. Rapid light rise to full output, from the dim level, is always possible simply by restoring full power to the lamp. The light rise time from the dim level will be a function of several factors. Firstly, the length of time the lamp has spent in the dim mode, as this affects the wall temperature thus affecting the overall energy input required for the lamp to again reach its final LTE arc stage. Secondly, the light rise time will be dependent on the cold start rise time. Lamps that take longer to reach full light output from the cold start will likewise take longer to reach full light output from the dim level. For Sodium Scandium based metal halide electrodeless discharge lamps, the light rise time from the dim level, typically 10% of the steady state light output, has never been observed to be longer than the cold start light rise time.
If an electrodeless HID lamp is fully extinguished it must cool to nearly room temperature before it can be restarted with microwave power alone. This cooling time, even for small lamps may exceed several minutes. A time period when no light is obtainable from the lamp is unacceptable in certain applications. In these instances hot restart for electroded lamps is typically achieved through application of high voltage pulses which provide electrical breakdown of the hot high pressure gas mixture within the lamp. For microwave driven electrodeless HID lamps, hot restart by conventional techniques is problematic due to the absence of electrodes.
Hot restart of electrodeless HID lamps by conventional techniques is sometimes achievable by supplying high voltage pulses to the lamp at a high frequency. When sufficient conductivity is produced within the arc tube by the pulses, the discharge may be picked up and maintained by the microwave field. Generation of such pulses and their application to compact low wattage electrodeless HID lamps may not be practicable. In such situations the present method and circuit may provide an alternative to hot restart, or at least, provide a means for dimming the lamp in a stable manner to a prescribed light level.
U.S. Ser. No. 07/626,365, filed Dec. 12, 1990, describes a method for controlled dimming of an electrodeless HID lamp to a predetermined light level at a predetermined rate. Although such a method has been successfully employed, it has been found that certain disadvantages do exist. For example, such a method has the disadvantage of requiring the user to re-adjust circuit settings for optimal performance on each individual lamp and to set the circuitry to promote lamp stability during dimming for the worst case of initial plasma conditions. Satisfying these criterion with a circuit based on a fixed light drop and decay rate often demands a compromise between optimal light response and reliable control performance.
To overcome the difficulties mentioned above, the present invention proposes a power control means for electrodeless HID lamp dimming which provides lamp stability regardless of the initial conditions of the discharge. In addition, the present invention provides a dimming means more forgiving of light-source-system manufacturing tolerances than is presently available, e.g., lamp to lamp variations as well as variations in the microwave coupling structure which applies the field to the lamp.